An example of an appropriate use of a fluid heater is described with reference to a temperature control unit for a semiconductor wafer treating solution shown in FIG. 10. In FIG. 10, a treating bath 100 is filled with treating solution 200. The treating solution 200 is forced to flow in a teflon piping 400 by a pump 300, heated by a fluid heater 500, filtered by a filter 600 to remove foreign substances contained in the treating solution, and returned to the treating bath 100. The temperature of the treating solution 200 in the treating bath 100 is detected by a sensor 700, and the controller 800 controls the fluid heater 500 so that the detected temperature approaches a preset temperature. In FIG. 10, numeral 510 denotes a box which incorporates the fluid heater 500, and numeral 900 denotes semiconductor wafers.
FIGS. 11(A) and 11(B) are respectively a perspective view and a sectional view of a conventional fluid filter. Specifically, a plurality of electric heaters 2 are provided on an external periphery of a fluid heating pipe unit 1 to heat fluid C introduced from an inlet pipe 31 and discharged through an outlet pipe 32. In this example, the fluid heater 2 comprises, in order from the inside, a fluid heating pipe unit 1, a clearance 7, an electric heater 2 formed with eight parallel members such as nichrome wire, kanthal wire and others, and insulation material 4.
In this case, the above described conventional fluid heater includes a problem as described below. The fluid heater heats fluid by radiant heat and, in this case, a temperature increasing rate of fluid to be heated by radiant heat is proportional to an energy density of absorption wavelength band of heated fluid (penetration energy in a unit time in a unit area) if a radiation area is fixed, and is proportional to the radiation area if the energy density is fixed. In other words, the energy density needs to be increased or the radiation area needs to be expanded in order to raise the heating rate.
By the way, the higher the temperature of the electric heater 2 is, the higher the energy density can be increased according to the Stefan-Boltzmann law. However, unless the energy is absorbed only by the fluid heating pipe unit 1 when the electric heater 2 is set to a high temperature, the temperature of the electric heater 2 rises, resulting in melting loss. Otherwise, even though the radiation area is expanded by increasing the number of members of the electric heater 2, the electric heater 2 is short-circuited, resulting in melting loss. In addition, part of the radiant heat is radiated outwardly from the external periphery of the electric heater 2 and absorbed by the insulation material 4 and therefore it does not contribute to the heating of the fluid.
On the other hand, the radiation area can be expanded to raise the heating rate but a larger fluid heating pipe unit 1 is required and the fluid heater needs to be a larger size. It is difficult to conduct heat to the portion of the fluid which flows in the central part of the fluid heating pipe unit 1, and consequently the heating rate is not raised.
Particularly in manufacturing semiconductor wafers, chemical solutions such as super-aqua-ammonia, sulfuric acid, hydrochloric acid and hydrofluoric acid are heated up to approximately 50.degree.-150.degree. C. in the fluid heater 500 for use in cleaning, etching and removing resist, but technologies in relation to corrosion resistance of the fluid heater 500 and a low degree of contamination of heated chemical solution are unknown.
For example, an embodiment disclosed in the Patent Application Disclosure No. 116246-1986 is a fluid heater differing from the above described prior art. Though not shown, the configuration of this embodiment is such that a fluid heating pipe unit is provided on the external periphery of the electric heater. Specifically, this fluid heater comprises an electric heater serving as an infrared radiation member and crystal glass forming an internal pipe of the fluid heating pipe unit which are integrated. In such configuration, when the infrared radiation member is to be replaced, substances adhering to new components are brought into the passage. For cleaning new components, the passage is exposed to the outside and there is a possibility of foreign substances which may intrude into the passage from an external atmosphere even though new components are cleaned. A treating solution for wafers made of silicon or the like in semiconductor device manufacturing process needs to be filtered in a clean room to remove foreign substances contained in the solution, and therefore work accompanying intrusion of foreign substances into the passage should be avoided. In addition, a chemical solution always leaks whenever the fluid heater is mounted and demounted, and the leakage can adhere to other components and an operator's body to result in a cause of corrosion and hazard to health. To prevent such leakage, the chemical solution should be removed in advance from the fluid heating pipe unit, which is a troublesome work. Fluids for etching and removing the resist are contaminated due to stains of devices after repeated use and therefore should be periodically renewed by replacing them with fresh fluids. In this case, the internal tube of the fluid heating pipe unit, which has a high temperature, is directly exposed to a low temperature unheated fluid and thus is subjected to a large thermal impact, and this will therefore be a cause of remarkable reduction of the service life. This tendency is further increased in the configuration shown in FIGS. 11(A) and 11(B). In other words, the electric heater 2 surrounded by insulation material 4 maintains a high temperature for a long time even after the current supply has been stopped. The inventors of the present invention have confirmed that, if fluid C is removed and new unheated fluid C is introduced into the heating pipe, this new fluid C is boiled on the surface of the heating pipe 1 to produce a great volume of vapor and the fluid heating pipe unit 1 would be broken by thermal impact due to boiling and vapor pressure.